Recently, in applications such as label package doubling as a protection of a glass bottle and a plastic bottle etc, and display of articles, cap sealing and accumulation package, there have been, widely used as a shrink label, a polyester-based heat-shrink able film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance. The use amount of the polyester-based heat-shrinkable film tends to increase being accompanied by an increase in volume of PET (polyethylene terephthalate) bottle containers etc.
Heretofore, a heat-shrinkable film with a thickness of 40 to 60 μm has been used as a label for covering a PET bottle. However, heat-shrinkable films used for packaging become mere garbage after using the contents. Given this situation, in response to the growing environmental awareness of reducing garbage as much as possible, film manufacturers are also attempting to reduce the thickness of a heat-shrinkable polyester film.
However, when the thickness of the heat-shrinkable polyester film is reduced, the temperature of the film is rapidly raised even inside of the film by heating, and hence the heat shrinkage of the film also rapidly begins. At this time, if there is a variation in thickness or molecular orientation in the width direction of the film, a significant variation in heat shrinkage properties occurs. As a result, an appearance defect, of the film after shrinkage may occur, leading to a defective product.
Conventionally, a film drawn in the width direction in a transverse drawing apparatus is known to have a different orientation state of molecules between the central part and the end parts of the film (bowing phenomenon). Patent Document 1 discloses a technique in which the bowing phenomenon is suppressed in a biaxially drawn film, which is not a heat-shrinkable film, by changing a film temperature in the width direction of the film when drawing in the width direction. Patent Document 2 also discloses a technique in which heating is performed such that a temperature distribution in the width direction becomes constant to obtain a transversely drawn film having no irregularity in thickness in the width direction.